The present invention relates to a method and apparatus for increasing accuracy in non-dispersive infrared analyzer systems for determining the concentration of a preselected gas in a homogeneous sample gas of spectrally overlapping constituents, and more particularly to a method and apparatus for determining the concentration of an interferent gas and removing its effect on the measurement of a preselected constituent gas in a gas sample.
Prior art has disclosed many methods and apparatus for measuring concentration content of a preselected component in a sample gas, the most spectrally specific of which is a non-dispersive infrared analyzer utilizing a pneumatic detector.
While pneumatic detectors charged with the component of interest will give generally accurate indications of the concentration of the selected component, a problem is many times encountered where other gas components present in the sample stream have absorption bands which either overlap or lie within the major absorption band of the selected components. For example, carbon dioxide has an infrared absorption band which overlaps the major infrared absorption band of carbon monoxide while water vapor has a minor absorption band which lies within the major absorption band of carbon monoxide. Thus, these gases, when present, interfere with the measurement of carbon monoxide by non-dispersive infrared techniques. In order to compensate for the infrared energy absorbed by water vapor and carbon dioxide, prior art has used additional pneumatic detectors specifically sensitized to determine the concentration of carbon dioxide and/or water vapor and in turn subtract the effect of these concentrations from the concentration measured by the carbon monoxide detector. This solution to the problem results in a very complex, bulky, and expensive carbon monoxide detectors for testing the sample gas. In order to accurately measure the concentration of a gas such as carbon monoxide, an additional pneumatic detector is required for each interferent to be measured, resulting in a multiple in the cost of a detector to measure one gas.
It is therefore an object of the present invention to provide a simple dual beam, non-dispersive infrared analyzer utilizing a single pneumatic detector which measures the component concentration in a sample gas with increased accuracy.
It is another object of the present invention to provide an inexpensive accurate measurement of a component gas in a sample gas stream.
It is a further object of the present invention to provide a gas concentration analyzer which has a high degree of accuracy.